<canvas width="600" height="400"> </canvas>
<script >
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = 'GhostWhite';
pincel.fillRect(0, 0, 600, 400);
pincel.fillStyle ='red';
pincel.fillRect(0,0, 200, 400);
pincel.fillStyle ='green';
pincel.fillRect(400, 0, 200, 400);
</script>